It is not within the scope of this chapter to explore all the possible modulatory mechanisms of hair production, cycling, and maintenance; however, mention should be made regarding some overarching themes that are well established to give a broad understanding of the conditions in which hair biology is regulated under normal conditions. First, there are sex, age, and ethnic differences in the production of hair. In men, facial and pubic hair follicle biology is regulated in part by androgens,
and these changes are considered some of the hallmarks of puberty. Hormonal changes in the physical properties of hair have also been noted in postmenopausal women where the ratio of estrogens to androgens is altered resulting in thicker more noticeable facial hair (Wines 2001). With age, the most noticeable effects are the loss of hair in men and loss of pigment in both sexes. Additionally, upon close observation, there are distinct phenotypic differences in the thickness and shape of hair fibers among the different ethnic groups (Wolfram 2003). For the most part, however, these differences are not determined by diet but by genetics, and the differences do play a role in the detection of trace elements, metals, and toxins acquired through ingestion.